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Ungulate migration is an important but increasingly rare ecological process. Although Yellowstone National Park (YNP) retains a rich assemblage of migratory ungulates, changing ecological conditions across its boundaries are posing new and complex challenges to our understanding of migratory populations. This study is focused on the Clarks Fork elk herd, a partially-migratory population whose migratory subpopulation spends it summers in high-elevation alpine and subalpine habitats inside YNP’s northeastern boundary. In the past decade, productivity of migratory elk has declined sharply while the non-migratory herd segment has remained stable or grown on private lands east of YNP. Early findings have revealed that pregnancy of migratory elk is exceptionally low, suggesting that YNP migrants are nutritionally stressed. Although fat gain in temperate ungulates is conventionally thought to be limited by summer forage quality, and the park has lately experienced prolonged drought, some have suggested that physiologically-costly, wolf-induced antipredator behaviors are reducing elk condition and reproductive performance. We are investigating the relative influence of habitat conditions versus antipredator behavior on the body-fat levels and reproduction of Clarks Fork elk. Our results will help YNP, the Wyoming Game and Fish Department (WGFD), the US Fish and Wildlife Service (USFWS), and other state and federal management agencies understand and manage elk populations and their migrations in the post-wolf reintroduction era.